### A sum becomes 4 times at simple interest in 10 years. What is the rate of interest?

A. 10% B. 20% C. 30% D. 40% Answer: Option C
$\begin{array}{l}\text{I}^{\text{st}}\ \text{method:-}\\ \text{Let rate is R%}\\ \text{Now, P}=100\\ \text{A = 400}\\ \text{I = 400}-100=300\\ \text{Time, T}=10\ \text{years}\\ \begin{array}{l}\text{I}=\frac{\text{PRT}}{100}\\ \text{ Or, }R=\frac{100\times I}{PT}\\ \text{ Or, }R=\frac{100\times300}{100\times10}\\ \therefore\text{R}=30\%\\ \end{array}\end{array}$
$\begin{array}{l}\text{II}^{\text{st}}\ \text{method:-}\end{array}$ Here, the sum become 4 times that means 100, become 400. Rate of such question is given by; $R=\frac{\text { interest }}{\text { time }}=\frac{300}{10}=30 \%$